The office has, for many years, been a stressful environment due, in part, to the large number of objectionable noise generators, such as typewriters, high speed impact printers, paper shredders, and other office machinery. Where several such devices are placed together in a single room, the cumulative noise pollution may even be hazardous to the health and well being of its occupants. The situation is well recognized and has been addressed by governmental bodies who have set standards for maximum acceptable noise levels in office environments. Attempts have been made by office machinery designers, in the field of impact printers, to reduce the noise pollution. Some of these methods include enclosing impact printers in sound attenuating covers, designing impact printers in which the impact noise is reduced, and designing quieter printers based on non-impact technologies such as ink jet and thermal transfer.
The low cost personal typewriter is purchased primarily for home usage (including both personal and in-home office) and for school usage. It is particularly desirable in these environments to reduce the acoustic noise level of the printing mechanism at the source to levels which are unobtrusive. For example, in the home, other members of the family should not be distracted by the clatter of typing if conducted in common rooms. In a secondary school or college setting, colleagues and others should not be disturbed if the user types in a library, a study hall or a dormitory room. Heretofore such usage has not been possible because typewriters are notoriously noisy devices. The silent operation of our low cost quiet typewriter will enable such usage because silence transports such useful appliances into new physical settings and enhances portability. A derived benefit will be freer communication among work group members as the user is able to work directly in the group in a non-irritating manner.
The industrial typewriter market segment is at the high end of the product cost continuum, i.e. in the $1000 to $2000 range. Thus, the incremental increase in manufacturing costs necessitated by numerous design changes represents a relatively small percentage of the product cost which is passed on to the ultimate purchaser. At the opposite end of the product cost continuum, i.e. in the $150 to $300 range, there is the consumer, or commodity, market. Clearly, any modification necessitated by the implementation of a sound reduction design will of necessity be extremely low in cost because the incremental increase in product cost to the consumer will not warrant a large percentage rise in this market.
An explanation of noise measurement is appropriate to explain the following statements regarding noise abatement achieved by our invention. Noise measurements are often referenced as dBA values. The "A" scale, by which the sound values have been identified, represents humanly perceived levels of loudness as opposed to absolute values of sound intensity. When considering sound energy represented in dB (or dBA) units, it should be noted that the scale is logarithmic and that a 10 dB difference equals a factor 10, a 20 dB difference equals a factor of 100, a 30 dB equals a factor of 1000, and so on.
Typical typewriters generate impact noise in the range of 65 to just over 80 dBA. These sound levels are deemed to be intrusive. For example, the IBM Selectric ball unit generates about 78 dBA, while the Xerox Memorywriter generates about 68 dBA, and the low cost Smith Corona Correcting Portable generates about 70 dBA. When reduced to the high 50s dBA, the noise is construed to be objectionable or annoying. It would be highly desirable to reduce the impact noise to a value in the vicinity of 50 dBA. The low cost typewriter of the present invention has been typically measured at about 50 dBA. This represents a dramatic improvement on the order of about 100 times less sound pressure than present day low cost typewriters, a notable achievement toward a less stressful environment.
The major source of noise in the modern typewriter is produced as the hammer impacts and drives a character pad to form an impression on a receptor sheet. Character pads are carried upon and transported past a print station at the ends of the rotating spokes of a printwheel. When a selected character is to be printed, it is stopped at the print station and the hammer drives it against a ribbon, the receptor sheet and a supporting platen, with sufficient force to release ink from the ribbon onto the receptor sheet.
In conventional ballistic hammer impacting typewriters a hammer mass of about 2.5 grams is ballistically propelled by a solenoid actuated clapper toward the character/ribbon/paper/platen combination. After the hammer hits the rear surface of the character pad, its momentum continues to drive it toward and against the ribbon/paper/platen combination and to deform the platen surface. Once the platen has absorbed the hammer impact energy it seeks to restore its normal shape by driving the hammer back to its home position where it must be stopped, usually by another impact. This series of high speed impacts is the main source of the objectionable impact noise in these printers.
Typically the platen deformation impact is very short, on the order of 100 microseconds duration. Intuitively it is known that a sharp, rapid impact will be noisy and that a slow impact will be less noisy. Thus, if the impact duration were slowed it would be possible to make the device quieter. In low end typewriters with printing speeds in the 10 to 12 character per second range, the mean time available between character impacts is about 85 to 90 milliseconds. More of that available time can be used for the hammer impact than the usual 100 microseconds. If, for example, the platen deformation time were stretched to even 5 to 10 milliseconds this would represent a fifty to one hundred-fold increase, or stretch, in the impact pulse width. It is also intuitive that in order for a slow impact to deform the platen by the same amount, for releasing the ink from the ribbon, a larger hammer mass (or effective mass) must be used. This is because manipulation of the time domain of the deformation changes the frequency domain of the sound waves emanating therefrom, so that as the impulse deformation time is stretched, the sound frequency (actually a spectrum of sound frequencies) emanating from the deformation is proportionately reduced and the perceived noise output of the lower frequencies is reduced. Since this is a resonant system, the mass will be inversely proportional to the square of the frequency shift. Therefore, a one hundred-fold increase in the time domain (100 microseconds to 10 milliseconds) will proportionately reduce the frequency output when a ten thousand-fold increase in the mass is effected. Clearly it would not be practical to increase the actual mass of the hammer by such a factor. As an alternative to increasing the hammer mass per se, its effective mass may be increased by means of a mechanical transformer.